A NEW BURGESS SHALE-TYPE DEPOSIT FROM THE EDIACARAN OF WESTERN MONGOLIA

Preservation of soft-bodied organisms is exceedingly rare in the fossil record. One way that such fossils are preserved is as carbonaceous compressions in fined-grained marine sedimentary rocks. Named after arguably the most famous example, such deposits of exceptional preservation are known as Burgess Shale-type (BST) deposits. Near the beginning of the Phanerozoic, during the early and middle Cambrian Period, BST deposits are more common and provide a critical view of early animal evolution. The earliest definitive fossil evidence for macroscopic animal-grade organisms, however, is found in the preceding Ediacaran Period, at the tail end of the Precambrian. BST deposits from the Ediacaran are much rarer, with only three previously-known examples: two diverse biotas from the Yangtze Platform of South China, the Lantian and Miaohe biotas, and a single multicellular algae specimen from Nevada, USA. Despite the possible evolution of bilaterians just before or during the Ediacaran based on molecular clocks, no conclusive evidence for animals has been found in these deposits. More fossil evidence from Ediacaran BST deposits might help constrain the timing and nature of early animal evolution. We have discovered a new Ediacaran BST deposit with exceptional preservation of non-mineralizing macro-organisms from the Tsagaan Oloom Formation in Zavkhan Province, western Mongolia. This fossil assemblage, here named the Tsagaan Oloom biota, currently consists of at least three forms of macroscopic multicellular benthic algae. One of these forms, with curly bifurcating laminae, dominates the biota and its fragments are preserved in uncountable abundance. All of these fossils are preserved as carbonaceous compressions in a thinly bedded black shale facies between two thick carbonate units. Work is ongoing to analyze the fossils in this biota using SEM-EDS, to better constrain their age using stable isotope geochemistry of stratigraphically adjacent carbonates, and to examine the geochemistry of the fossil-bearing strata. Future excavations should reveal more exceptionally preserved fossil species and this biota stands to provide important information regarding the early evolution of multicellular organisms, perhaps including animals.